Electrostatic recording is the process of producing an image in the form of an electrostatic charge pattern on a dielectric surface and subsequently developing that latent image by toning with oppositely charged black or colored powder, usually colloidally suspended in an insulating liquid.
In a typical writing or imaging process, a writing head, which contains two or more rows of densely spaced styli and a backplate or frontplate electrode, is selectively programmed by the plotter logic to place minute dot-spaced electrostatic charges in latent image form on the recording medium. This medium is designed to receive and hold an electrostatic charge pattern. After the latent image is electronically placed on the medium, the medium is exposed to a liquid toner. Black or colored particles suspended in the toner vehicle adhere to the medium only where a previous electrostatic charge was placed. Excess toner is removed from the medium by a vacuum channel or wiper bars and the medium is then dried by forced air, thereby fixing the image to the medium. This electronically produced print is often referred to as a hardcopy.
Although various recording media have been proposed for use with electrostatic recording plotters or printers, none of them has satisfied fully the substantial need in the art, particularly for use with electrostatic recording devices such as the Benson (Oce Graphics) plotter, Versatex VS 3000 Series, 7000 Series, 8500 Series and 8500-HR Series plotters, CalComp 5700, 5800, 67000 and 68000 Series plotters and HP 7600 Series plotters. In fact, there are many deficiencies in the known products which have considerably limited their commercial utility.
Some of the main image defects which often are experienced in electrostatic recording are artifacts or flares, image breakup or dropouts and glitches. Flares are randomly occurring bursts or explosions in plotter lines due to abnormal electrostatic discharge. Dropouts are defined either as missing dots in continuous lines or in single dot patterns (half tones), or as dots missing in a specially recorded quadrant, consisting of 51 rows of 50 single dots in a row, as in a test plot performed in a CalComp 68436 color plotter. Glitches are irregular specks or non-uniform images occurring in solid dark images due to irregularities in the dielectric surface of the media.
Another defect commonly seen is mechanical and electrical nib writing which causes dark areas on the image.
Additional image problems may occur with conventional recording media because of humidity changes during use since the media is inherently moisture sensitive. This sensitivity to changes in humidity may result in repeated images or lines (ghosting) and low image density.
Various attempts to provide adequate electrostatic recording media have been made, none of which have satisfactorily solved the above problems while providing a clear, dense image.
U.S. Pat. No. 3,657,005, issued to Clevite Corporation, deals with the need to establish a controlled gap between the flat surface of a charging device and the area on the surface of the electrographic recording media where the latent image is formed. The '005 patent also specifies the frequency of the spacers per certain square area, such as 1 to 10 spacer means per 100 square rail of surface, with the spacers projecting from 0.05 to 0.4 mil above the recording media. Glass shot, starch, refractory particles and plastic particles are recommended as spacers. The simple mechanistic approach of designing a 3-D imaging space, as in '005 patent, addresses only the requirements imposed by the physics of latent charge formation on the flat surface of the dielectric materials. However, the '005 patent does not deal with humidity independent conductive layers and does not describe any composites containing carriers other than paper based media.
U.S. Pat. No. 4,752,522, issued to Mitsubishi Rayon Co., deals with the application of spacers using an insulating resin of specific volume resistivity at a narrow average particle size from 1.5 to 4 .mu.m and limiting particles of 8 microns and more in size to 0.02% by number in the distribution. The '522 patent describes the use of certain types of polymeric materials such as copolymers of acrylonitriles and styrene as the spacer particles and puts a limitation on the design of the electrostatic media both in terms of the materials to be employed and in limiting the upwards protrusion height in a range similar to U.S. Pat. No. 3,657,005. The only quality issues which are addressed are line dropouts, pen writability and image density and most of the findings in this patent are specific only to acrylonitrile-styrene copolymers used as spacers.
U.S. Pat. No. 4,795,676, issued to Oji Paper Co., deals with a composite dielectric material, including a support, that is formed of a multilayered sheet of synthetic paper and which has an electroconductive layer and a dielectric layer. The support has a surface layer formed of a thermoplastic resin film containing 0-3 wt. % of an inorganic fine powder and a paper-like layer which is made from a thermoplastic resin film containing from 8 to 65% by weight of inorganic filler (fine powder). The paper-like layers are formed on both sides of a stretched film base. The support contains no more than 50 elevations per 0.1 m.sup.2 that project upwards for 10 microns or more from the flat side of the surface layer. Despite the cumbersome structure of the composite design, image density of the hard-copy resulting from the composite of the '676 patent does not exceed 1.1-1.2 units of optical density, which is only a minimum acceptable density. Further, material produced by the '676 patent is made with a humidity sensitive conductive base and possesses all of the negative properties related to the fluctuations of the resistance of a conductive ground plane with the change of relative humidity.
U.S. Pat. No. 5,116,666, issued to Fuji Photo Film Co, deals with a composite consisting of insulating film, a conductive layer and a dielectric layer, which includes conductive fillers in the form of fibrous conductive powders.
U.S. Pat. No. 5,130,177, issued to Xerox Corp., discloses a conductive coating composition consisting of a quaternary ammonium compound deposited on one side of the paper and a dielectric coating composition on the other side of the paper. The basic objective of the '177 patent is to provide a paper suitable for electrostatic recording which has minimum curl, is less humidity dependent and is flexible. The '177 patent connects paper stiffness (flexibility) with the image quality in order to improve line dropout. However, as taught in column 12, lines 25 to 35, the surface resistivity changes from 0.11 Mohm/sq at 71% R.H. to 2.5 Mohm/sq at 24% R.H., an order of magnitude change which is totally unacceptable, for example, when opaque matte or transparent electrographic film is to be manufactured.
U.S. Pat. No. 5,192,613, issued to E. I. du Pont de Nemours and Company, introduces a humidity insensitive electroconductive layer having a conductive powder consisting of amorphous silica or a silica-containing compound surface coated with a two dimensional network of antimony-containing tin oxide crystallites. Both dielectric and conductive layers are the same cross-linked polymers, having a preferred powder to polymer ratio of about 0.6 to about 1.3. The use of polyfunctional aziridines such as PFAZ.RTM. 322 or XAMA-7, containing residual highly toxic and dangerous ethyleneimine is an undesirable feature of the design. Another disadvantage of the '613 patent is the relative hiding power of the core or shell design for conductive powders, which makes it difficult to use for the design of transparent hard copy.
U.S. Pat. No. 5,194,352, issued to Dai Nippon Printing Co., teaches a toned image of high fidelity by a contact between two films, one electrostatic and another photosensitive, positioning both of them within a boundary of a predetermined gap, and imaging the electrostatic layer via exchange exposure through the transparent photosensitive layer and then submerging the film with developed latent image into a toner. When both films are submerged into a toner, positive and negative images are obtained. Thus, the '352 patent requires two types of media, one known as media for transparent electrophotography (TEP) and the other one conventional dielectric imaging material.
U.S. Pat. No. 5,126,763, issued to Arkwright Inc., describes a multilayer polymeric film composite for use in the electrostatic recording process. It specifies a surface abrasivity of about 0.015 inch to about 0.085 inch and a surface roughness of about 30 to about 180 cc of air/minute and teaches the use of an electronically conductive layer containing at least one electronically conductive particulate in at least one polymer binder deposited beneath the dielectric layer with the specified roughness. This design has many significant deficiencies, which were discovered during intensive studies by the inventors. Among them are a gray cast of the film, an extremely excessive electrical background, when material is imaged especially, but not only, on Capricorn Plotters, as for example, CalComp 68436 series, a high level of artifacts expressed as flares, and a wider than optimal line width. Moreover, the '763 patent teaches the use of crystalline silica. The presence and use of materials known to cause lung cancer and silicosis such as crystalline silica is clearly unacceptable at the present level of medical and environmental science. Additionally, the abrasion level advocated by the '763 patent is detrimental to the state of the print head as it is gradually abraded away by hard abrasive pigments, such as crystalline silica.
Though the above patents address many questions in latent image formation, print quality and favorable interaction of media and plotter, there is still a need in the art to provide a media material which can store a highly intense latent image charge, provide a high image density after toning when Dmax is above 1.3, and provide good grounding conditions for the return of writing current to the electrical ground. A desirable media material will also employ a humidity independent conductive ground plane for opaque, matte and transparent paper, vellum, film and fabric and provide an optimized air gap for the Paschen discharge. Further desirable properties include minimization of flares and dropouts and prevention of filming, including aid in cleaning the print head of polymeric deposits and inorganic oxides. Other desirable properties include high toner adhesion, good archival characteristics and use of safe chemical compounds and procedures, avoiding noxious or smelly material, toxic chemical, carcinogenic or mutagenic compounds.
Thus, there is a need in the art to provide a high performance, high speed dielectric recording media substantially free of the disadvantages, defects and limitations of the materials disclosed in the art.
An object of the present invention is to provide a technology which utilizes unique and appropriate chemicals, compounds and procedures which will result in the manufacture of high fidelity, high resolution electrostatic imaging material.
Another object of the invention is to produce an electrostatic imaging material with smallest possible dot size, i.e., a high resolution product.
Yet another object of this invention is to produce a material which shows little or no dropouts.
Another object of the present invention is to provide a humidity insensitive transparent film of a high quality electrostatic recording.
Another object is to provide a matte film of a high quality electrostatic recording.
Another object of this invention is to provide a humidity insensitive white paper or film product for electrostatic recording of high quality.
A further object of this invention is to provide a matte film for a high quality electrostatic recording which consists of a dielectric layer, a conductive layer, a transparent polyester support and a matted antistatic coating.
An additional object of this invention is to provide low flare electrostatic recording media.
Another object of this invention is to produce a media with a diminished number of white spots. A white spot is a circular untoned area with a diameter greater than 1 mm.
Yet another object of this invention is to provide a composite for dielectric recording with high thermal stability, allowing full removal of residual solvents.
A further object of this invention is to provide a dielectric layer having low abrasion characteristics, as defined by a brass shim abrasion method, described below, as well as by the method described in U.S. Pat. No. 5,126,763, in order to provide less abrasion to the print head.
A further object of this invention is to provide a dielectric recording media of archival quality, i.e., with high resistance to scratching, excellent toner adhesion, high moisture resistance, etc.
A further object of this invention is to provide a high quality material, with a low level of electrical background, i.e., with an imperceptible nib writing level.
Still another object of the present invention is to provide a conductive ground plane which can utilize extremely low levels of dopant for the electronically conductive particulate.
Yet another object of the present invention is to provide a white ground plane which exhibits a conductivity in the range of from 1 to 10 Mohms/sq. Still another object of the present invention is to provide conductive ground planes in various colors.
Yet another object of the present invention is to provide a novel impingement process for the preparation of an electrically conductive layer wherein the electronically conductive particulate is reduced to a nanoscale size.
These and other objects of the present invention will become apparent upon review of the following specification, the drawing and the claims appended thereto.